Sorry for your loss.
Different IPs work in slightly different ways, but the theory is that, during the IVA you still owe the balance of the original debt, plus fees. They can also add statutory interest to this ( though sometimes this is excluded in the paperwork). This is at 8% p.a on the reducing debt.
The inheritance would be treated as a windfall and, as such, would be payable into the IVA up to whatever the outstanding amount, fees and possible interest is.
I have seen some IP's allow for a full and final offer to be paid with inheritance funds --- a full and final offer is based on the total of remaining payments and is usually somewhat less than the full settlement figure of debt / fees / interest.
Clear as mud, I am afraid, but you will need to chat to your provider to see which way they are likely to jump.
I am not qualified to give advice and can only state my opinions, based on my IVA experiences.